People often choose smaller rewards over larger rewards when the smaller reward is available sooner. Similarly, people often choose a smaller reward over a larger reward when the smaller reward has a higher probability of receipt. Behavioral economic explanations for such behavior are based on temporal discounting (i.e., the decrease in subjective value of a future reward as delay to its receipt increases) and probability discounting (i.e., the decrease in subjective value of a reward as the probability of its receipt decreases), respectively. Temporal discounting functions describe relations between subjective value and delay, and different models assume different forms of discount function. Similarly, probability discounting functions describe relations between subjective value and the likelihood of obtaining a reward. The proposed research will evaluate different discounting theories and their implications for the nature of the choice process. Animals as well as humans will be studied in order to evaluate the generality of the theories and to address issues difficult to study experimentally with humans. People will choose between hypothetical monetary and non-monetary rewards of different amounts, probabilities, and delays;animals will be given similar choices between different amounts and types of food. Temporal and probability discounting play important roles in decision making, and may explain such behavioral problems as gambling and addiction. The proposed research addresses the question of whether temporal and probability discounting represent two distinct, albeit related, phenomena, or whether one provides the basis for the other. Finally, temporal discounting is central to current psychological models of self control. That is, self control represents the ability to defer immediate rewards so as to receive greater, delayed rewards. Differences in impulsivity and self control, then, are a consequence of differences in the steepness of temporal discounting functions. In addition, risk aversion may result from probability discounting, and differences in risk-taking may reflect differences in the shallowness of the probability discounting function. Theories of self control based on temporal and probability discounting may help explicate situational, individual, and developmental differences in self control, and the proposed research may help strengthen the empirical and conceptual foundations of such theories.